This invention relates to semiconductor devices such as metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) and to a method of fabricating such semiconductor devices.
Semiconductor devices have been known which use gallium-nitride-based compound semiconductors, examples being MESFETs and HEMTs.
In a typical prior art semiconductor device using gallium-nitride-based compound semiconductors, the compound semiconductor region is formed on an electrically insulating substrate of sapphire via a low-temperature buffer layer of GaN or AlN, the latter having been formed with a relatively low substrate temperature of approximately 500° to 600° C.
More specifically, in the case of a MESFET, a working layer known as the channel layer of silicon-doped n-type GaN is formed on an insulating sapphire substrate via a low-temperature buffer layer of GaN or AlN. The source, drain and gate electrodes are formed on the surface of the working layer.
In the case of a HEMT, on the other hand, an electron transit layer or channel layer of undoped GaN and an electron supply layer of n-type AlGaN are formed on an insulating sapphire substrate via a low-temperature buffer layer of GaN or AlN. The source, drain and gate electrodes are formed on the surface of the electron supply layer.
One of the problems encountered with the sapphire substrate is its hardness, which made dicing very difficult and inefficient. Another problem is the expensiveness of sapphire, which added substantively to the manufacturing costs of the semiconductor devices concerned.
Still another problem with the sapphire substrate is its low heat conductivity, 0.126 W/cm·K. Incapable of sufficiently liberating the heat developing during operation of the device, the sapphire substrate deteriorated the performance characteristics of the device such as voltage-withstanding capability and gain.
A further problem manifested itself in conjunction with the heterojunction of the prior art HEMT constituted of GaN and AlGaN layers. As a result, the AlGaN layer inherently possessed a tensile stress or tensile strain due to lattice mismatch between the GaN and AlGaN layers. The tensile stress gave rise to piezoelectric depolarization at the interface between the GaN and AlGaN layers. Combined the spontaneous depolarization, the piezoelectric depolarization produced an electric field of the order of several megavolts per centimeter at the heterojunction interface between the GaN and AlGaN layers. The electric field created in turn two-dimensional electron gas of the order of 1013 cm−2 in the channel layer, causing a decrease in the sheet resistance of the channel layer and, in consequence, an increase in the amount of drain current. The reduced sheet resistance of the channel layer due to the two-dimensional electron gas is an admitted strength of the HEMT having the GaN—AlGaN heterojunction.
Running counter to the noted advantage of the HEMT is the fact that the sapphire substrate is easy to expand thermally, sapphire being higher in coefficient of thermal expansion than nitride semiconductors. This difference in thermal expansion coefficient gave rise to compressive stress on the epitaxially grown layers. Such compressive stresses lessened the piezoelectric depolarization by working in a direction to cancel the tensile stress in the AlGaN layer due to its lattice mismatch with the GaN layer. The result was a drop in the electron density of the two-dimensional electron gas in the channel layer, preventing the GaN-AlGaN heterojunction HEMT from exhibiting its strength to the full.
In an attempt to solve the problems discussed above, both Japanese Unexamined Patent Publication No. 2001-274376 and Japanese Patent Application No. 2001-248735 suggest use of a silicon substrate in lieu of a sapphire one. The second recited reference further teaches the compositions of a buffer layer to be formed on the silicon substrate, in order to create a nitride semiconductor region of favorable crystallinity on the buffer layer. The buffer layer taught is a lamination of two different kinds of sublayers. One of these kinds of sublayers is fabricated from a class of substances generally defined as:AlxMyGa1-x-yNwhere M is at least either of indium (In) and boron (B); 0<x≦1; 0≦y<1; and x+y≦1. The other kind of buffer sublayer is fabricated from a class of substances generally defined as:AlaMbGa1-a-bNwhere M is at least either of In and B; 0≦a<1; 0≦b≦1; a+b≦1; and a<x.
The first kind of buffer sublayer with its relatively high aluminum content has a lattice constant intermediate the lattice constants of silicon and nitride semiconductors. As a consequence, with the nitride semiconductor region formed on the silicon substrate via the laminated buffer sublayers as taught by the prior art, the buffer layer faithfully conforms to the crystal orientation of the silicon substrate. The nitride semiconductor region also faithfully conforms to the crystal orientation of the buffer layer.
However, when a buffer layer is constituted of alternations of an AlN or AlGaN sublayer and a GaN sublayer, two-dimensional electron gas layers are created at the heterojunction interfaces between the AlN or AlGaN sublayers and GaN sublayers. These two-dimensional electron gas layers are so low in resistance that the HEMT having a buffer layer of such alternating sublayers has current paths through the buffer layer in addition to the drain current path through the channel layer. The current paths through the buffer layer provide leakage current paths between the source and drain when the gate is off, inviting a rise in current leakage which is unnecessary in semiconductor devices. Furthermore, the voltage withstanding capability of semiconductor devices depends upon current leakage. A semiconductor device is less in voltage withstanding capability the greater its current leakage. Current leakage will lessen if the electron transit layer of undoped GaN has fewer crystal defects. As of today, however, no practical measures are available for reduction of crystal defects in the electron transit layer. It might be contemplated to render the electron transit layer higher in resistance either by making this layer thicker or by doping with a p-type impurity the electron transit layer of GaN which possesses an n-like conductivity type even though it has been undoped. These methods are objectionable because the gallium-nitride-based semiconductors would be so stressed as to develop cracks, resulting in poor performance of the semiconductor devices.
The above discussed inconveniences with current leakage manifest themselves with semiconductor devices other than HEMTs.